1. Field of the Invention
The invention relates to bistable twisted nematic LCDs, and particularly to optimization of operating modes thereof.
2. Description of Related Art
Bistability of liquid crystal twist cell was first discovered by Berreman. It was not until 1995 that the first prototype Bistable Twisted Nematic (BTN) cell was demonstrated (Tanaka et. al.). The demonstrated cell was based on a xcfx80(180-degree) twist configuration. The cell gave high contrast ratio, fast response, wide viewing angle, and a high cross-talk immunity. Since then, different cell configurations have been proposed. Among them, BTN cells with 90xc2x0 and 270xc2x0 initial twists were proposed by Xie et al. Despite the poorer optical performance of those proposed non-180xc2x0 twist cells, they have other advantages such as larger operating retardation value. All the proposed cell configurations adopt twist angles that are multiples of 90. The reason is that based on the assumption of linear crossed polarizers, the transmission is theoretically optimized at twist angles of 90xc2x0 multiple. However, it is obvious that the dark state breed-through is also important in determining the optical performance of a BTN cell. In fact, in the design of a display, instead of the bright state transmission, the contrast ratio is a more important factor to be optimized. Besides, to obtain an achromatic display, it is imperative that we take the whole visible spectrum into consideration. We observe that the performance of BTN cells can be further enhanced if the assumption of cross polarizers is removed.
Using two definitions of contrast ratios (one favours cells with high transmission while the other favours cells with low breed-through) as objective functions, optimized cell configurations covering two ranges of twist angles are proposed. The lower-twisted group offers higher retardation that relaxes the small cell gap requirement. Adopting larger cell gaps eases the manufacturing process and results in a better cell gap control. Hence, the ratio of cell gap to helical pitch (d/p), which determines the stability of the BTN select states, can be better controlled. The higher-twisted group outperforms the existing BTN configurations with less dark state breed-through. Cells with the proposed configurations are fabricated. They show excellent optical performance, with high contrast, wide viewing angle, and achromatic appearance.
It is an object of the invention to seek to provide a BTN cell of enhanced performance.
According to the invention there is provided a bistable twisted nematic LCD cell, comprising front and rear polarizers and a liquid crystal layer having an initial twist angle, wherein the cell does not have a configuration with a twist angle of 180 degrees and an angle between the polarizers of 90 degrees.